Niche Theory is a central framework in ecology based on the recognition that most interactions between organisms are indirect, mediated by the biotic and abiotic dynamical environment these organisms live in. Despite its potential generality, the theory still mostly focuses on how resource–consumer dynamics mediate competition in ecological communities. However, it is being increasingly recognized that positive interactions between organisms also play an important role in driving the structure and functioning of ecological communities, from plants to microbes. A unified theory of the niche is presented, that applies to both positive and negative interactions between organisms, mediated by one or two environmental factors.Classical concepts such as niche differences and fundamental and realized niches can naturally be expanded to facilitative and mutualistic interactions. New general niche concepts that appear exclusively in the presence of positive interactions are: (1)the Allee niche, a region of environmental conditions for which a species can persist but not invade from low densities and (2) niche facilitation, when the presence of a species expands the set of environmental conditions under which a second species can invade and/or persist. The broad applicability of this theory is illustrated by these concepts using a diverse set of theoretical examples, from bacteria feeding on an inhibiting substrate, to nitrogen-fixing plants and the indirect mutualism between a plant and a carnivore species. In sum, Niche Theory provides a natural framework for positive interactions in ecology, bringing a unified perspective and new conceptual tools to study ecological systems where these positive interactions occur.
This paper advocates for the development of a general theory of species interactions. More particularly, the framework of Niche Theory specifically tailored to indirect interactions mediated by explicit environmental factors, shows how a concept like niche difference, usually used to quantify the strength of competition, can equally be applied to situations that involve positive interspecific interactions. This approach suggests building on the connection between lower-level metrics of direct interactions between the focal species and their environment (the sensitivity and impact niche differences) and an effective, indirect metric of niche difference. With several possible options to pick from for the latter, we identified pros and cons for each metric are identified, which suggests that there is no ideal metric to measure effective niche difference.